1. Technical Field
The invention relates to a toner for development of an electrostatic image, an electrostatic image developer, a toner cartridge, a process cartridge and an image forming apparatus.
2. Related Art
Methods of visualizing image information via an electrostatic image, such as an electrophotographic method, have been employed in various fields. In the electrophotographic method, an electrostatic image is formed on a photoreceptor through processes of charging and exposing to light, and is visualized by developing with a developer containing a toner, transferring and fixing.
The toner mentioned above is generally composed of toner matrix particles containing a binder resin, a colorant, a releasing agent, a charge control agent and the like, which are formed into particles by a kneading pulverizing method, a suspension polymerization method, an emulsion aggregation method, a dissolution suspension method, or the like; and an auxiliary agent that is added to the surface of the toner matrix particles, such as inorganic metal oxide particles of silica, titania, alumina or the like, and inorganic/organic particles that are optionally added to aid cleaning capacity or polishing capacity of the photoreceptor. Further, with both black and white printing and full color printing, a toner usable with oil-less fixing devices in which oil is not supplied to a fixing roll, serving as a fixing member, has been widely used.
In the aforementioned electrophotographic process, various kinds of mechanical stresses are applied. Therefore, to stably maintain the functions of the toner, it is necessary to suppress exposure of a releasing agent to the surface of the toner and, further, it is necessary to enhance surface hardness and fixing ability of the toner itself in order to improve mechanical strength and maintain sufficient chargeability. Additionally, in response to the demand for high image quality, the size of the toner has been remarkably reduced in order to realize a highly precise image in an image formation process.
However, simply reducing the size of the toner without changes to conventional particle size or shape distribution results in toner particles having minute sizes or deformed shapes, which could cause problems such as contamination of a carrier or a photoreceptor with the toner, scattering of the toner, or attachment of the toner to a fixing roll rather than a recording medium. Therefore, it is difficult to achieve both of high image quality and high reliability. Consequently, there is a demand for a toner having both particles of reduced size and narrower particle size distribution or shape distribution.
Further, there has been a demand for a technique by which a toner may be fixed with less energy in order to reduce energy consumption of a copier or a printer, and therefore a toner for electrophotography that can be fixed at lower temperature has been strongly desired.
As a means for reducing the fixing temperature of the toner, a technique of lowering a glass transition temperature (Tg) of a resin for a toner is widely employed. However, if the glass transition temperature is too low, aggregation of toner powder (blocking) may easily occur or storability of the toner formed on a fixed image may be lost. Therefore, the glass transition temperature has to be about 50° C. at lowest, in practical use.
The use of polyester resin as a binder resin has been attempted due to its superior low-temperature fixability and heat-resistant storability, in place of styrene and acrylic resins that have been widely used as binder resins. However, there is a problem with polyester resins that dispersibility of a releasing agent (wax) in the polyester resin is poor and the mixture tends to pulverize at an interface of the binder resin and the releasing agent, thereby causing degradation of toner powder characteristics or charging characteristics due to the exposed releasing agent on the toner surface. Moreover, even in a wet method including aggregation and coalescence processes, there has been a problem that degradation of toner powder characteristics or charging characteristics is caused by the releasing agent that tends to be exposed on the toner surface, or detach from toner particles, at the time of coalescence process carried out with heat.